In the past three decades, electricity has risen from 25% to 40% of end-use energy consumption in the United States. With this rising demand for power comes an increasingly critical requirement for highly reliable, high quality power. As power demands continue to grow, older urban electric power systems in particular are being pushed to the limit of performance, requiring new solutions.
Wire forms the basic building block of the world's electric power system, including transformers, transmission and distribution systems, and motors. The discovery of revolutionary HTS compounds in 1986 led to the development of a radically new type of wire for the power industry; this discovery is the most fundamental advance in wire technology in more than a century.
HTS-coated wire offers best-in-class performance, carrying over one hundred times more current than conventional copper and aluminum conductors of the same physical dimension do. The superior power density of HTS-coated wire will enable a new generation of power industry technologies. It offers major size, weight, and efficiency benefits. HTS technologies will drive down costs and increase the capacity and reliability of electric power systems in a variety of ways. For example, HTS-coated wire is capable of transmitting two to five times more power through existing rights of way. This new cable will offer a powerful tool to improve the performance of power grids while reducing their environmental footprint. However, to date only short samples of the HTS-coated tape used in the manufacture of next-generation HTS-coated wires have been fabricated at high performance levels. In order for HTS technology to become commercially viable for use in the power generation and distribution industry, it will be necessary to develop techniques for continuous, high-throughput production of HTS-coated tape.
The structure of HTS-coated tape consists of a polished metal substrate that provides strength and flexibility supporting an HTS film formed of, for example, rare-earth-barium-copper-oxide (REBCO). One or more buffer layers are disposed between a polished metal substrate and the HTS film to prevent reaction between the substrate and the HTS film and to provide a template for the epitaxial growth of the film. The buffer layer may be formed of, for example, yttrium-stabilized zirconia (YSZ) and/or cerium oxide (CeO2).
The techniques that exist for the manufacture of REBCO-coated tapes can be classified as in-situ or ex-situ processes. In-situ processes encompass those in which film growth occurs entirely in one place, as when a vapor containing rare-earth, barium, and copper precursors reacts with oxygen at the surface of a heated substrate to form the REBCO film. In-situ techniques include sputtering, electron beam (e-beam) evaporation, and pulsed laser deposition (PLD) processes, each of which occurs in a single low-pressure oxygen atmosphere such as a vacuum chamber.
On the other hand, ex-situ techniques encompass deposition processes that occur in more than one step, separated in time and often in space as well, as precursors are deposited atop a substrate and subsequently undergo a separate post reaction that converts the precursors to a REBCO film. The precursors may be initially deposited atop the buffered substrate via a number of processes known to the art, including electron beam evaporation, a coating step such as dip coating in metalorganic deposition (MOD), and spray pyrolysis.
In electron beam evaporation used in an ex-situ process, evaporation occurs to three separate crucibles containing rare-earth, barium fluoride, and copper metals. In the MOD process, triflouroacetic acid (TFA) complexes of rare-earth, barium, and copper are mixed with a solvent such as methyl alcohol, and the resulting solution is applied to the buffered substrate in a dipcoating process under ambient conditions, and the dipped substrate subsequently undergoes a bakeout process in which the organics are baked off the substrate. The dip coating and bakeout steps are then repeated a number of times until the desired film thickness is achieved.
In spray pyrolysis, nitrates of rare-earth, barium, and copper form an aqueous precursor solution that is atomized and sprayed atop the heated buffered substrate. In spray pyrolysis, the spray and bakeout steps occur simultaneously by heating the substrate at moderate temperature, for instance, 500° C. that is not high enough to form superconducting REBCO phase.
Gross, et al. U.S. Pat. No. 5,416,063, dated May 16, 1995 and entitled, “Method of Producing a Layer of Superconductive Oxide,” provides a method for forming a superconducting layer on a buffered substrate in which a precursor solution is applied to the substrate such that a metal-containing layer is formed on the surface. The precursor solution described in the U.S. Pat. No. 5,416,063 patent is formed by dissolving rare-earth-, barium-, and copper-containing compounds in acetic acid and water. However, as is the case with all ex-situ HTS film growth techniques, a post process is required to convert the metal-containing layer atop the buffered substrate to a superconducting film.
The post process may be a water vapor reaction, in which the substrate is heated and water vapor is applied to and reacts with rare-earth, barium fluoride, and copper metals contained thereon to form the REBCO film. However, the post reaction occurs very slowly, with film growth on the order of only 1 Angstrom per second, when compared with in-situ REBCO film growth techniques, in which film formation occurs in a single step and at a rate of up to 1-5 microns per minute, such as with PLD. It remains a challenge to provide an ex-situ REBCO film growth system well suited for the high throughput necessary to enable cost-effective production, and hence widespread adaptation of HTS materials in the electricity transmission/distribution industry. As a result, the method described by the U.S. Pat. No. 5,416,063 patent is not well suited for the manufacture of long lengths of HTS tapes.
One of the limitations of the present technologies is that, as an attempt to grow thicker and thicker REBCO films is made, it becomes increasingly difficult to force the water vapors to penetrate the dense deposited layer consisting of precursors of rare-earth, barium, and copper.
It is crucial to the attainment of high-quality HTS tapes that the water vapor penetrates deeply into the deposited precursor layer for film growth to occur from the bottom of the deposited precursor layer up, such that the REBCO film is grown epitaxial to the buffer layers and acquires the desired texture. When the REBCO film growth occurs from the top of the deposited precursor layer down, the nucleation and subsequent growth of the polycrystalline grains that comprise the REBCO film occurs randomly, with a high degree of grain boundary misalignment that severely compromises the current-carrying capability of the HTS film, as opposed to a biaxial texture that assures a high current-carrying capability.
It is further a challenge to effectively exhaust the reaction byproducts of the REBCO film production process which limit the reaction kinetics and inhibit REBCO film growth.
It is therefore an object of the invention to provide a process for producing thick high quality REBCO films.
It is an object of the invention to provide a process that provides for removal of reaction byproducts that limit the reaction kinetics and inhibit REBCO film growth.
It is a further object of the invention to provide a high-throughput ex-situ REBCO film growth system.